Wood pulp digestion



Aug. 14, 1.951 A. N. PARRETT WOOD PULP DIGESTION Filed Jan. 17, 1946 Patented Aug. 14, 1951 2,564,028 woon PULP mGEsTIoN Arthur N. Parrett, Shelton, Wash., assignor to Rayonier Incorporated, Shelton, Wash., a corporation of Delaware Application January 17, 1946, Serial No. 641,717

3 Claims.

This invention relates to wood pulp digestion, and has for its object the provision of an improved method of digesting wood for the production of cellulose fiber. More particularly, the invention aims to provide an improved sulfite process for Wood digestion capable of producing widely diierent types of sulte wood pulp, and especially adapted to produce a special type of pulp for use in the production of cellophane.

In the manufacture of chemical wood pulp by the sulte process, the digesting liquor (usually called cooking acid) consists of anaqueous solution of sulfur dioxide (or sulfurous acid) and a bisulte which may be the bisulte of calcium, sodium, ammonia or some other less common metallic bisulte. The sulfur dioxide combined in the form of bisulte is termed the combined SO2" (but is actually expressed as 1/2 of the amount of sulfur dioxide combined as bisulfite), and the sulfur dioxide dissolved or present as sulfurous acid is termed the free SO2. Sullite digesting liquors may vary from 0.3 to 2% combined SO2 and from 3 to 15% free SOg. In commercial practice, the free SO2 usually lies between 5 and 14% and the combined SO2 between 0.8 and 1.8% depending on the type of pulp being made. Y

In normal commercial sulflte digester operation, it is common practice to relieve the digester during the cook. The top relief is to permit sulfur dioxide gas to pass oil as the temperature is increased. The side relief is to permit removal of a portion of the acid during the latter part of the cook for several purposes, one of which is to permit more complete gassing oi. Since the acid relieved through the side relief contains a considerable amount of dissolved organic matter (mostly lignin sulfurous acids and sugars) and since this acid is added to the acid recovery tanks, cooking acid normally contains from .3% to 1.21% organic matter inv addition to the free and combined sulfur dioxide.

The residual liquor associated with the digested or cooked pulp is commonly known in the industry as "Waste suliite liquor, and while various attempts have been made to utilize this liquor, it is generally run to waste in streams or rivers. A particular object of the present invention is the provision of an improved method of manufacturing sulte wood pulp in which the waste liquor is economically disposed of by combustion or incineration. A further object of the invention is to effect waste sulte liquor recovery and disposal with a minimum amount of equipment. Other objects of the invention will 2 Dpear in the vcourse of the following descripion:

In accordance With the invention, wood chips are digested or cooked with an ammonium base sulte liquor containing from l0 to 40%, and preferably at least 20%, of the total solids present in the waste sulte liquor from a prior similar digestion. To this end, such an amount of Waste sulfite liquor is rst recovered or separated from the digested Wood pulp that from 10 to 40% (preferablyrat least 20%) of the solids of the total Waste liquor Vremains associated with the digested pulp. The Waste liquor thus first recovered or separated frdm the digested pulp is subjected to concentration by evaporation in which the required heat is furnished by combustion of concentrated liquor. Following the recovery or separation of such waste liquor, the digested pulp is washed, preferably rst with a final Weak wash water from a prior similar operation, and then with fresh water. The dilute 'waste suliite wash liquor, resulting from the first Washing step, is utilized in making up ammonium base'sulflte cooking acid for a subsequent similar digestion, and the nal Weak wash water is utilized for the rst washing of the digested pulp from a subsequent similar digestion. The waste sulte liquor (of digester strength) may advantageously contain from 12 to 17%, and preferably around 14%, of total solids. The solids content of the rst recovered Waste liquor usually suffers from dilution with Water in the course of blow pit washing since such operations are rarely if ever Y% ecient. Consequently the solids content of the rst Waste liquor recovered for evaporation and subsequent combustion will usually be from 11 to 15%, and preferably around 13%. The freshly made-up ammonium base sulfite cooking acid may contain 2 to 9%, and preferably 3 to '7%. of solids derived from the dilute Waste sulte wash liquor of a prior similar digestion. The total solids of cooking acid or of waste sulfite liquor may be dened as the dry residue which remains after heating a liquor sample to constant weight at C. The total solids content naturally will contain some inorganic matter such as ammonium salts and inorganic compounds liberated from the Wood.

The invention, in its preferred and complete aspect, is thus characterized by (1) digestion of the wood chips with an ammonium base sulfite cooking liquor or acid, (2) the inclusion in that liquor of a predetermined amount of ammonium base waste suliite liquor solids, and (3) disposal of the ammonium base waste sulte liquor by combustion or incineration. More specifically, the method of the invention comprises the following sequence of steps:

1. Preparation-of an ammonium base sulflte cooking Aacid containing at least 20% of the total waste sulfite liquor solids resulting from a preceding sulte pulping operation.

2. Digestion of wood chips in a digester with this ammonium base cooking acid according to conventional or desired sulflte pulping practice.

3. Recovery of ammonium base waste sulfite liquor at the highest possible solids content, as, for example, by draining off digester-strength liquor from the digested pulp in a blow pit, and by replacing the residual liquor in the bed of the pulp in the blow pit successively by weak wash water, and finally by freshwater.

4. Evaporation of the recovered Waste sulfite liquor in a breeching type evaporator to a solids content between 25 and '70%.

5. Combustion of the evaporated (concentrated) waste sulflte liquor in a refractory furnace into which the liquor is sprayed, the gaseous products of combustion being used to supply the required heat for the evaporator.

While the foregoing sequence of steps contemplates' the recovery of the waste suliite liquor at a concentration close to that at which it occurs in the digester as in step 3, the recovery may be effected in other suitable ways, as for example by the conventional practice of using counter-current vacuum washers. The choice of washing equipment will usually be dictated by the prevailing conditions at any particular location. Concentration by evaporation of the recovered waste liquor may likewise be effected in other than a breeching type evaporator. Moreover, the practice of the invention is not intended to limit or restrict variations in sulflte pulping practice as established in the industry to produce sulte pulps of widely different characteristics as required commercially for many different purposes. Likewise, theoperation of making an ammonium base sulflte cooking acid may be practiced in equipment commonly used for making calciu'm base sulte cooking acid, except of course, that no other base, such as calcium, is deliberately introduced into the ammonium base vsulite cooking acid of the invention.

The amount of ammonium base waste sulflte liquor solids included in the digesting or cooking acid of the invention is determined by the desired heat balance in the subsequent operation of washing the ammonium base waste sulte liquor from the pulp and evaporating the waste liquor by the heat evolved in the combustion of the concentrated liquor. The amount of waste liquor added to the cooking acid determines the total solids in the Waste liquor as it is blown from the digester. 'Ihe efficiency of the washing operation must then be such that the recovered waste liquor will, after evaporation, provide enough heat to concentrate the recovered waste liquor to a solids content practical for combustion. Thus, it is important that the ammonium base waste sulte liquor be recovered at a concentration such that the heat of combustion of the evaporated or concenrated liquor provides a flow of hot gases sufficient to evaporate all of the recovered liquor. Under such conditions no waste liquor remains to be otherwise disposed of. To accomplish this purpose, the solids content of therblow liquor is built up by including waste liquor solids in the cooking acid, and the desired heat balance is obtained by the proper adjustment of the amount of waste liquor solids added to the cooking acid. The ammonium base makes it possible to carry out this sequence of steps without encountering the difficult problem of disposing of large quantities of ash such as would be formed in the combustion of the conventional waste sulflte liquor resulting from the use of a calcium base sulte cooking acid.

In practicing the method of the invention, liquid ammonia, as received in tank cars, is run as a liquid into water and absorbed to form a strong solution of ammonia which is stored in a tank under a slight pressure. The strong solution of ammonia is injected into the dilute waste sulfite wash liquor (recovered from the blow pit and containing from 10 to 40% of the total solids present in the waste liquor of a preceding pulping operation), as that liquor'is pumped to the sulphur dioxide absorption towers, where sulphur dioxide is absorbed to form a cooking acid with the desired ratio of free and combined SO2. This cooking acid will have a relatively high content of organic solids, because of the inclusion therein of a predetermined amount of waste sulte liquor from a prior digestion. I have found that cooking acids made in this way serve effectively for the manufacture of high grade sulte pulps, and that cooking practices can be varied as desired to produce pulps of varied quality and characteristics, and in particular a special type of pulp for use in the production of cellophane.

Since the waste suliite liquor must be recovered for evaporation and combustion as an integral part of the process, it is necessary to provide suitable means for so doing. This may advantageously be accomplished by draining the :aste sulfite liquor (of digester strength) from the pulp in aconventional type of blow pit, washing the bed of pulp in the blow pit, iirst. with dilute wash water, and, finally, with fresh Water. The volume of fresh water and hence the volume of dilute wash water will be approximately the same as the volume of cooking acid introduced into the digester, due account being taken of the volume of water used for absorption of liquid ammonia. and of the volume of liquor returned to the cooking acid storage tanks by way of wet relief or liquor drawn from the digesters during the course of cooking. The drainings from the blow pit are separately collected so that the strong waste sulte liquor may be diverted div rectly to the evaporator, and the dilute waste sultelwash liquor used in making-up cooking acid for the deliberate purpose of increasing the total organic solids of the waste sulfite liquor as it comes from the digester.

The strong waste sulfite liquor is pumped to a horizontal spray evaporator, which may consist of any desired combination of disk or cascade evaporators and/or spray chambers. The liquor is evaporated to a solids content of 25 to '70%, a liquor of 35% solids content burning very satisfactcrily. The evaporated liquor is burned in a refractory furnace in which the liquor is introduced'v as a spray together withv the required quanity of air for combustion, and the resulting hot products of combustion pass through' the evaporator. Finally, the gases passing from the evaporator are disposed of through a chimney, preferably of a height suiilcient to disseminate the gases in the atmosphere without nuisance to the surrounding community. Thus, the waste sulte liquor is completely disposed of in a practical and economical manner.

The single iigure of the accompanying drawing is a. diagrammatic flow sheet of an equipment suitable for the practice ofthe invention. The arrangement of the equipment and its operation will be clear from the legends on the drawing taken in conjunction with the following description.

The digester may be of conventional design. A pump serves to introduce the required amount oi ammonium base sulfite cooking liquor into the digester, suitably filled with wood chips, and subsequently to circulate the cooking liquor through a heater and the digester. In general, the incinerator is an elongated furnace structure interiorly divided by transverse bridges into a combustion chamber and a plurality of successive spray evaporating chambers. The combustion chamber is fired by downwardly directed fuel burners provided with steam jets adapted to spray the concentrated sulte liquor into the combustion chamber. The strong waste sulflte liquor recovered from the blow pit is delivered to a transverse head box having a perforated bottom above and registering with a transverse slot in the roof of the furnace, and showers downwardly through the first or-hottest evaporating chamber approximate its hottest end. In the operation of the evaporator liquor falls as a spray through each evaporating chamber, collects in apool at the bottom of each chamber and ows into an external sump from whence it is pumped to transverse head boxes with perforated bottoms above and registering with transverse slots in the roof of the respective chambers. From the sump of the last or coolest evaporating chamber, the resulting evaporated or concentrated liquor is pumped to the burners and sprayed into the combustion chamber, where it burns much like fuel oil in an atmosphere'of preheated air blown into the chamber along with the liquor spray.

The following examples illustrate practices 0f the invention:

An ammonium base sulte cooking acid was prepared as hereinbefore described from dilute sulte wash liquor recovered from the blow pit washing system. This dilute liquor contained about 20% of the total solids present in the Waste sulte liquor from a prior pulping operation, and was of approximately the volume required as Y cooking liquor for a pulping operation. The

tained about 14% solids. This recovered liquor used in acid make-up cannot be greater than the volume of the made-up cooking acid.l Under conditions where total waste liquor disposal is desired, it is necessary to use all of the dilute liquor for acid make-up unless the heat balance permits inclusion of some of the dilute liquor in the strong liquor pumped to the evaporator and combustion unit. The total solids contained in the dilute liquor will depend partly on the eiliciency of washing in the blow pits, and, in turn. on the amount of waste liquor solids returned to the acid make-up. This amount may lie between 10 and 40% of the total solids in the blow liquor which would occur if a. cooking acid were used which did not contain any liquor solids. At the start-up of such an operation, water must be used for acid making if nov ammonium base dilute waste sulte wash liquor is available. The waste liquor produced on the start-up cooks are relatively low in solids content because (1) no side-relief has functioned concentratedliquor was burned to furnish the hea/tY 'ljiired for evaporation.

Infanother example, the ammonium base sulte cooking acid, similarly prepared, contained about 35% of the total solids present in the Waste sulte liquor of a prior pulping operation. The

solids content of this cooking acid was about l Some dilution of,

to add solids to the cooking acid and (2) no dilute waste liquor has been used as acid make-up. As the operation proceeds, the side-relief and dilute waste liquor usage start to build up solids in the cooking acid, and, by the same token, in the blow liquor. This continues until an equilib- .rium value is reached. For example, the addition to the acid make-up of 20% of the total volume of the blow liquor as dilute waste liquor would add 2.0% solids to the blow liquor from the rst digester blown, assuming that the blow liquor from the start-up blow contains 10% solids. Two factors would then work to increase the solids of the blow liquor to a certain equilibrium value, namely (1) a gradual increase in the solids contained in 20% of the total volume of blow liquor produced and used for making cooking acid, and (2) a gradual increase in the solids content of the liquor returned to the cooking acid storage tanks by way of wet relief or liquor drawn from the digesters during the course of cooking. After equilibrium has been reached, the total solids of the blow liquor would rise to about 13.0% with the continued use for acid make-up of the dilute waste liquor containing 20% of the solids of the precedingblow liquor and the continued removal of liquor from the digesters during the course of cooking and addition of this liquor to the cooking acid storage tanks. With blow liquor of about 13% ofsolids, the solids content of the recovered liquor for diversion to the evaporators would be somewhat lower depending on the eiiiciency of the blow pit washing operation hereinbefore described.

I claim:

1. The method of wood pulp digestion which comprises digesting Wood chips with an am-v monium base suliite cooking acid containing from 10 to 40% of the total solids present in the Waste liquor from a prior similar digestion, the waste liquor at the end of digestion containing from 12% to 17% of total solids, separating from the digested pulp from 60% to 90% of the waste liquor at substantially digester strength, washing the digested pulp containing the balance of the waste liquor to recover in the resulting wash liquor the bulk of the residual waste liquor remaining associated with the pulp. utilizing said wash liquor in making up ammonium base sulte cooking acid for a subsequent similar digestion, subjecting said separated waste liquor to concentration by evaporation, burning oo ncentrated waste liquor from a preceding like digesting operation, and utilizing the heat of said burning operation to supply the total heat for said evaporation.

2. The method of Wood pulp digestion which comprises digesting wood chips with an ammonium base sulte cooking acid containing at least 20% of the total solids present in the waste liquor from a priox` similar digestion, the waste liquor at the end of digestion containing from 12% to 17% of total solids, separating from the digested pulp up to a maximum of 80% of the waste liquor at substantially digester strength, washing the digested pulp containing the balance of the Waste liquor to recover in the resulting wash liquor the bulk of the residual waste liquor remaining associated with the pulp, utilizing said wash liquor in making up ammonium base sulite cooking acid for a subsequent similar digestion, subjecting said'separated waste liquor to concentration by evaporation, burning concentrated waste liquor from a preceding like digesting operation, and utilizing the heat of said burning operation to supply the total heat for said evaporation.

3. The method of claim 1 in which the separated waste liquor is concentrated to a solids content of from 35 to 70%.

ARTHUR N. PARRETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,100,519 Brech June 16. 1914 1,130,192 Oman Mar. 2, 1915 10 1,723,800 Michael Aug. 6, 1929 1,803,738 Thomsen May 5, 1931 l1,811,268 Haglund June 23, 1931 1,880,047 Richter Sept. 27, 1932 1,909,929 Bradley May 23, 1933 2,032,437 Richter Mar. 3, 1936 2,320,294 Palmrose May 25, 1943 2,385,259 Collings Sept. 18, 1945 FOREIGN PATENTS Number Country Date 389,218 Great Britain Mar. 16, 1933 OTHER REFERENCES Chemical Abstracts. vol. 36 (1942), page 3044. Paper Trade Journal, August 28, 1941, pages 34 thru 38.

Technical Association of the Pulp and Paper Industry Papers, Series 15 (1931-32), pages 372 thru 376. 

1. THE METHOD OF WOOD PULP DIGESTION WHICH COMPRISES DIGESTING WOOD CHIPS WITH AN AMMONIUM BASE SULFITE COOKING ACID CONTAINING FROM 10 TO 40% OF THE TOTAL SOLIDS PRESENT IN THE WASTE LIQUOR FROM A PRIOR SIMILAR DIGESTION, THE WASTE LIQUOR AT THE END OF DIGESTION CONTAINING FROM 12% TO 17% OF TOTAL SOLIDS, SEPARATING FROM THE DIGESTED PULP FROM 60% TO 90% OF THE WASTE LIQUOR AT SUBSTANTIALLY DIGESTER STRENGTH, WASHING THE DIGESTED PULP CONTAINING THE BALANCE OF THE WASTE LIQUOR TO RECOVER IN THE RESULTING WASH LIQUOR THE BULK OF THE RESIDUAL WASTER LIQUOR REMAINING ASSOCIATED WITH THE PULP, UTILIZ- 